Center for Materials at Irradiation and Mechanical ExtremesA BES Energy Frontier Research Center

Irradiation Extremes Thrust

This movie shows our molecular dynamics simulation of a collision cascade near an asymmetric Σ11 tilt grain boundary in copper over a time of 380 ps. The grain boundary is at the center of the system. The top and bottom layers are fixed.

The atoms are colored by their energies, and only defects are shown, including those atoms in the grain boundary and fixed surface. A primary knock-on atom (PKA) with 4-keV kinetic energy is initiated 15 angstroms below the grain boundary and is directed toward the grain boundary. The PKA quickly induces a cascade (disordered region consisting of many radiation-induced interstitials and vacancies) that overlaps with the grain boundary. After the cascade cools down, most displaced atoms return to lattice sites, but two interstitials – one above and the other one below the grain boundary – remain in the bulk.

Note here an interstitial is characterized by a cluster of several atoms with a red core. Similarly, two vacancies also remain very close to the grain boundary. The interstitials are highly mobile and perform 3-D random walk in the bulk. Once they are about 10-15 angstroms from the grain boundary, they are quickly absorbed by the grain boundary, as shown in this movie. As a result, after the cascade only two vacancies remain in the bulk region of the material. Therefore, under the appropriate conditions, grain boundaries can be good sinks for absorbing radiation-induced defects during damage production.